1. Technical Field of the Invention
The present invention relates to the processing of incident signals in a signal receiver.
2. Description of Related Art
Receivers are known that combine several multi-path signal components mutually retarded by different time delays before reaching the receiver.
For example, one such receiver is present in Code Division Multiple Access (CDMA) wireless communication systems, that experts in the subject usually call a “RAKE” receiver.
In a wireless communication system, a base station communicates with several remote terminals such as mobile cell phones. Frequency Division Multiple Accesses (FDMA) and Time Division Multiple Accesses (TDMA) are traditional multiple access schemes to supply simultaneous services to a number of terminals. The basic idea on which the FDMA and TDMA systems are based is to share the available resource into several frequencies or several time slots respectively, such that several terminals can operate simultaneously without causing interference.
Telephones operating according to the GSM standard belong to the FDMA and TDMA systems in the sense that transmission and reception take place at different frequencies and also at different slots.
Unlike these systems that use a frequency division or a time division, CDMA (code division multiple access) systems enable multiple users to share a common frequency and a common timing channel, by using a coded modulation. CDMA systems include the CDMA 2000 system, the wide band CDMA (WCDMA) system, and the IS-95 standard.
As is well known to an expert in the subject, a scrambling code is associated with each base station in CDMA systems to make a distinction between one base station and another. Furthermore, an orthogonal code known to an expert in the subject as the “OVSF (Orthogonal Variable Spreading Factor) code,” is allocated to each remote terminal (for example like a mobile cell phone). All OVSF codes are orthogonal with each other, so that one channel can be distinguished from another.
Before transmitting a signal on the transmission channel to a remote terminal, the signal is scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the channel.
In CDMA systems, a distinction can still be made between systems that use a distinct frequency for transmission and for reception (CDMA-FDD system) and systems that use a common frequency for transmission and reception, but have distinct time domains for transmission and reception (CDMA-TDD system).
An incident signal, for example received by a mobile phone, comprises several versions of the initially transmitted signal with different time delays. These versions of the initial signal are the result of the multi-path transmission characteristics of the transmission medium between one base station and the telephone. It is recognized that each path introduces a different delay.
The “RAKE” receiver installed in a mobile cell phone operating in a CDMA communication system is used to make the time alignment, descrambling, despreading, the channel correction and the combination of delayed versions of the initial signals, so as to deliver information flows (symbols) contained in the initial signals.
All of these functions require the use of considerable memory means and/or a high operating frequency and large adders, particularly for the combination means of the receiver. The result is not only larger and more expensive equipment, but also higher power consumption.